Cmos output buffer having improved noise characteristics

Abstract

A CMOS output buffer circuit which as improved noise characteristics is disclosed. The circuit has two stages, one having relatively fast response time for causing the output node to make a quick logic transition, and the other stage for providing steady-state drive of the output node. The transistors in the transition-driving stage are driven from power supply and reference supply nodes which are isolated from the power supply and reference supply nodes of the steady-state stage. For a low-to-high transition, the driving transistor in the steady-state stage, being p-channel, drives the output node to a full power supply level, which causes the driving transistor in the transition-driving stage to turn off, isolating the two power supply nodes of the two stages from one another. For a high-to-low transition, a feedback circuit serves to turn off the pull-down transistor of the transition-driving stage in order to isolate the two reference supply nodes of the two stages from one another. The steady-state stage is delayed, so that the noise from the initial transition does not appear at the power supply and reference supply nodes of the steady-state stage. The circuit also operates so that the output node can be put into a high-impedance state, allowing for common input/output utilization of the circuit.